Evidence of DMS and other biogenic gases affecting iron bioavailability in remote marine aerosols

1. SO42- CCN OH• NO3• H3CCOCHO (g) SO2 (g) MSA (g) HCOCHO (g) MSIA (g) HOCCH2OR (g) DMSO (g) Fe2O3 (s) FeO(OH) (s) Fe(OH)3 (s) Feedback cycles ISOPRENE (g) DMS (g) Relevant aqueous phase reactions: CH3COCH0aq +2 HO˙→ (COO)22-aq (COO)22-aq + Fe(III)aq→ Fe(II)aq+ CO2 MSIA + Fe(III)aq→ MSA(aq) + Fe(II)aq DMS (aq) DMSP (aq) Fe(III)(aq) + Fe(II)(aq) Phytoplankton ISOPRENE (aq) 1 3 2 Figure 4: The cruise track broken into three distinct regions based on characteristic air mass back trajectories (AMBTs), created using NOAA’s GDAS database using the HYSPLIT model. 2 1 2 3 Figure 5: Representative Air Mass Back Trajectories. (Barone et al., 1996; Boyd et al., 2000; Charlson et al., 1987; Davis et al., 1998; Patroescu et al., 1999; Sunda et al., 2002; Turner et al., 1996; Turnipseed et al., 1996; Zhuang et al., 1992) R/V Kilo Moana Acknowledgements This research was supported by National Science Foundation Grant ATM-0137891 and Central Washington University. People: Dr. Jim Murray, CWU Atmospheric Chemistry Group High volume cascade impactor Evidence of DMS and other biogenic gases affecting iron bioavailability in remote marine aerosols Anne M. Johansen, Lindsey M. Shank, Mari N. Sorey, Matthew J. Lenington, Zhen Zhang, Brittany Best, Department of Chemistry, Central Washington University, 400 East University Way, Ellensburg, WA 98926, johansea@cwu.edu Abstract Iron availability limits open-ocean phytoplankton growth, and because phytoplankton account for half of the Earth’s photosynthesis they are key players in modulating global climate. Atmospherically transported dust particles provide an important source of iron to remote regions, yet, the mechanisms that control iron speciation and thus its bioavailability remain ill-defined. The present study pertains to elucidating processes that occur on atmospheric dust particles before deposition into the ocean. Laboratory experiments have identified a chemical link between iron reductive dissolution of synthesized iron(oxy)hydroxides and methanesulfinic acid (MSIA), an oxidation product of dimethyl sulfide (DMS) emitted by iron-starved phytoplankton. We present evidence for the existence of this mechanism in aerosol particles collected over the equatorial Pacific Ocean. Furthermore, results suggest that biogenically emitted isoprene oxidation products also affect iron speciation. These findings support the hypothesis that phytoplankton can actively affect iron availability through a direct biogeochemical feedback cycle. Results Geographic area designation based on aerosol chemical characteristics Organic Acids Labile Iron Introduction and Motivation Iron (Fe) is an essential micronutrient necessary for the metabolic processes (e.g., photosynthesis and cellular respiration) of marine phytoplankton. In open-ocean environments, far from continental shelves and upwelling currents, iron availability is limited to the atmospheric deposition of crustal-derived aerosols that have been transported to sea by prevailing winds [Gao et al., 2003; Talbot et al., 1990; Tegen et al., 2004]. It has been suggested that iron present on these particles undergoes chemical processing during long-range transport, which ultimately leads to an increase in the more soluble Fe(II), which is also thought to be the more bioavailable fraction. Laboratory simulation results reveal significant Fe(II) and MSA increases from a ligand to metal charge transfer (LMCT) between MSIA and Fe(III) on the surface of ferrihydrite (Figures 1-3). MSIA, an oxidation product of biogenically emitted DMS, has an effect at concentrations that correspond to atmospheric levels lower than 0.12 pmol m-3, four orders of magnitude less than typical concentrations of its precursor DMSO [Lee et al., 1999] and three orders of magnitude lower than observed concentrations of its oxidation product MSA [Johansen et al., 2000]. For ferrihydrite, the proposed reaction is prevalent at pH values < 4.2, which coincide with model predictions for aerosol pH in the remote marine boundary layer [Fridlind and Jacobson, 2000]. Since DMS is released from phytoplankton when under oxidative stress, such as caused by iron limitation or increased UV radiation [Sunda et al., 2002; Toole and Siegel, 2004], the proposed mechanism suggests a mechanism by which phytoplankton can actively affect iron availability through a direct biogeochemical feedback cycle [Zhang et al., 2006; Zhuang et al., 2003]. The mechanism identified here will help explain current discrepancies in marine atmospheric iron and sulfur models, where sources of Fe(II) and MSA have remained unidentified, respectively [Hand et al., 2004; Lucas and Prinn, 2002; Luo et al., 2005; von Glasow and Crutzen, 2004]. [Johansen and Key, 2006; Key et al., 2008] Sulfur Containing Compounds Regions 1 1 2 3 Principal Component Analysis Output for Equatorial Pacific Ocean Samples Figure 3: Photochemical experiments with one batch of ferrihyrdrite in the presence of varying amounts of MSIA. Reaction order with regard to adsorbed MSIA, a ≈ 1. Reaction constant, k = 1.4 x 10-4 s-1 Figure 2: Photochemical experiments with one batch of ferrihyrdrite synthesized from Fe(ClO4)3•6H2O in the presence of DMSO, DMSO2, MSIA, and MSA. a. Fe(II), b. Sulfur Species; MSIA (solid symbols) and MSA (corresponding open symbols), and c. H2O2. Initial and final pH values are noted near the end of each curve. Figure 1: Proposed Reaction mechanism between MSIA and Fe(III) Summary and Conclusions • Methods • Sample Collection • High volume cascade impactor (ChemVol 2000, R&P), 760 L/min, four size fractions: ultrafine (da≤0.1 μm), fine (0.1≤da<1 μm), coarse (1≤da<10 μm) and large (da≥10μm). Sample pretreatment, handling and storage were performed following strict trace metal clean procedures. Samples were stored at -20 oC until analysis was feasible. • Analysis of 152 high-vol samples • Ion Chromatography (IC): acetate, MSIA, formate, MSA, pyruvate, Cl-, NO2-, Br-, NO3-, malonate, SO42-, oxalate, and PO43-. Na+, NH4+, K+, Mg2+, and Ca2+; • Long pathlength absorbance spectroscopy with a long waveguide capillary cell (LWCC, WPI, 200 cm) and portable spectrometer (TIDAS): labile iron species = Fe(II)(aq) and easily reducible Fe(III) (with hydroxylamine); and • Inductively Coupled Plasma Mass Spectrometry (ICPMS): 37 trace metals. • Principle Component Analysis (PCA) was performed using SPSS v.15.0 and data was evaluated in the context of air mass back trajectories (7.5 day AMBT, NOAA HYSPLIT model [Draxler, 2002] with GDAS database). • Fine ferrous iron predominates in the pristine equatorial Pacific Ocean • This ferrous iron correlates with • MSA and malonate, (PC 3) and • Oxalate (PC 5). • These findings indicate that iron solubility in pristine areas is controlled by oxidation products of biogenically emitted gases thereby further supporting the hypothesis that phytoplankton and aerosol iron solubility are involved in a biogeochemical control cycle. 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